1. Field of the Invention
The present invention relates to an oscillator of a semiconductor memory device, and more specifically, to an oscillator of a semiconductor memory device, wherein the duration period of an internal control signal can be kept constant by generating a reference clock having a constant clock cycle regardless of variation in a power supply voltage.
2. Discussion of Related Art
In a semiconductor chip, an oscillator is used to generate a reference clock of a pump or a timer. The reference clock used for the timer plays an important role in deciding the duration period of a control signal used in the chip. As such, variation in the cycle of the reference clock has influence on specification of a product.
For example, in NAND type flash memory devices, variation in the cycle of the reference clock is closely connected with a read access time, which is a time taken to read data from memory cells. If the cycle of the reference clock reduces, the read access time reduces. In this case, however, there occurs a case where data of the memory cells are not exactly sensed because all control signals of an internal chip operation are reduced.
As described above, since the oscillator has a great effect on the operation of a chip, it must be designed to be less sensitive to variation in PVT (Process/Voltage/Temperature). In a conventional oscillator, however, a clock cycle frequently shifts according to variation in a power supply voltage. Therefore, it is very difficult to keep the duration period of internal control signals constant.